This invention relates to a resinous composition for a water-based paint (hereinafter referred to as water paint) particularly to a resinous composition for a water paint crosslinkable at the normal temperature suitable for painting of the interior and exterior of buildings, bridges, ships, vehicles and the like.
In water paints dryable by crossliking at normal temperature, an aqueous dispersion of an acrylic copolymer has been generally used. For the aqueous dispersion of an acrylic copolymer, emulsion particles are necessary to be fused and adhered to each other. Therefore, it is difficult to increase the Tg (glass transition temperature) of the resin as to the acrylic copolymer used in the water paint dryable at normal temperature due to its film forming property, so that its contamination resistance is lowered and its water resistance and the like also decreases.
On one hand, the market tends to require higher quality year by year, and properties such as paint film elasticity, toughness, contamination resistance, acidic rain resistance and the like equivalent to organic solvent type urethane paints have been increased also in water paints.
On the other hand, with the conventional organic solvent type urethane paints, it is necessary for these to be a suitable undercoat in order to withstand the effect of solvent used, otherwise it is difficult to apply the paint particularly in repainting use. Further, aqueous systems have been recently required because of envionmental hygienic aspects and the fire prevention laws, thus an aqueous polyurethane paint dryable at the normal temperature having the same properties as the organic solvent type urethane paint is strongly desired.
There have been carried out up to now some trials to blend an aqueous dispersion of a carbonyl group-containing copolymer with an aqueous dispersion of a urethane polymer having a hydrazide residue for the purpose of providing a urethane function to an aqueous emulsion (see Japanese Laid-Open Patent Nos. 1-301761/1989 and 1-301762/1989). These compositions, however, did not solve the problem of producing a strong film dryable at normal temperature, and did not possess the desired product properties of rapid drying, water resistance, as well as outdoor stain resistance and acidic rain resistance.
The inventors have previously proposed a water paint dryable at normal temperature in order to solve the above-mentioned problems, which paint is obtained by strenthening the film by using a comparatively low molecular weight crosslinking agent using together with an aqueous dispersion of crosslinkable urethane resin (Japanese Laid-open Patent No. 5-339542/1993). Also, applicants have proposed a composition wherein a polyurethane resin containing a carbonyl group obtained by the reaction of a diisocynate compound (d) with a glycol (e), an alcohol and/or a glycol having a carboxyl group (f) and an alcohol having a carbonyl group (g) is used as an aqueous polyurethane resin (Japanese Patent Application No. 5-205496/1993).
Although the above-mentioned paints has solved some problems of paint films, there remain some problems in safety and hygienic aspects due to use of hydrazine or aqueous hydrazine solution. Furthermore, the water paint according to Japanese Patent Application No. 5-205496/1993 is satisfactory in its paint film properties, but there is a problem of complexity in the procedure for its preparation.